1. Field of the Invention
The present invention relates to a drive mechanism for a closing element or stopper at the spout or discharge opening of a vessel containing molten metal.
2. State of the Prior Art
German Patent Document DE A1 31 35 048 discloses a known pouring level control for controlling the filling height of the metal melt in a continuous casting mold. In this document, a stopper is used as the closing element and is controlled by an operating cylinder. The stopper is controlled either through a servo valve connected to the operating cylinder or is moved manually by opening a bypass valve to connect opposite sides of the piston in the operating cylinder. The operating cylinder of this invention is designed as a differential cylinder, and the bypass valve and the servo valve can be rigidly connected as a subassembly to the operating cylinder.
Although the differential cylinder used in the above German patent document employs a simple method of construction, this fact is of minor importance when considered in the context of the operating environment. The distributing molten metal vessels that employ stoppers as closing elements do not have a problem with respect to the amount of space available for the provision of the operating cylinder.
However, the differential cylinder used in the above German Patent Document does have the drawback that the cross sectional areas of the cylinder chambers on the opposite sides of the piston in the cylinder vary in size. As a result, the control of the operating cylinder is complicated. Further, if the pressure lines are connected, as by the bypass valve, to connect the opposite sides of the piston for manual operation, either an excess or an insufficient amount of hydraulic fluid will flow from one side of the piston toward the other side of the piston. As a result, it is necessary that the hydraulic fluid either be returned to or drawn from the hydraulic fluid reservoir.
As noted above, the servo valve and the bypass valve can be rigidly connected together as one subassembly on the operating cylinder. However, this causes a weight increase of the operating cylinder. This weight increase is undesirable, because the operating cylinder frequently has to be removed from the molten metal vessel during operation.